1. Field of the Invention
The present invention relates to an AD conversion circuit which converts an analog signal into digital data and an error correcting method employed in such an AD conversion circuit.
2. Related Art
In general, various kinds of control such as engine control of an automobile are performed by a control microcomputer or a CPU (hereinafter referred to generically as “microcomputer) on the basis of output values of various sensors. Analog signals output from the sensors are converted by an AD (analog-to-digital) conversion circuit into digital data, which are input to the microcomputer. Sensor data which are bases of various kinds of control are required to be of high precision and the AD conversion accuracy is also required to be considerably high. For example, input and output voltages of batteries and motors used in electric vehicles and hybrid cars are up to several hundreds of volts. It is required to measure such a voltage with an error of several percent (e.g., 2%) or less and convert it into digital data.
Major factors that cause AD conversion errors include a characteristic of the AD conversion circuit and a variation of a power source voltage supplied to the AD conversion circuit. The error due to the characteristic of the AD conversion circuit is what is called a quantization error and generally measures ±1 to 3 LSBs. For example, in an AD conversion circuit having a resolution 10 bits and a reference voltage 5 V, an error of ±3 LSBs corresponds to ±14.7 mV, which means an error of about 1.47% when a voltage 1 V is measured.
Usually, AD conversion circuits quantize an input analog voltage using a unit voltage obtained by dividing a power source voltage or a reference voltage according to a resolution. Therefore, a variation of the power source voltage is reflected in a measurement value. For example, a power source voltage that is supplied to an AD conversion circuit used in a vehicle from a vehicular voltage regulator has a variation of about ±2%, and hence an output measurement value of the AD conversion circuit may also have an error of about ±2%.
In order to reduce the influence of such errors, conventionally, various methods for correcting an error of an AD conversion circuit have been proposed (refer to Patent documents 1 and 2, for example).    Patent document 1: JP-A-06-204868    Patent document 2: JP-A-2005-244771
In the AD conversion circuit disclosed in Patent document 1, a correction reference analog voltage is input to the AD conversion circuit and an AD conversion error is determined on the basis of a value obtained by converting the correction reference analog voltage into digital data and stored. When an input analog voltage is converted into digital data, the digital data is corrected by shifting it by the stored AD conversion error. In the AD conversion circuit disclosed in Patent document 2, correction values corresponding to respective input voltages are stored in a memory in advance and digital data produced by converting an input analog voltage is corrected using a corresponding correction value.
However, the above conventional methods have limits in the accuracy of AD conversion error correction and have difficulty increasing the correction accuracy further. More specifically, since errors due to the characteristic of an AD conversion circuit are not uniform in a voltage measurement range, errors remain when a single correction value is applied over the entire measurement range. One method for correcting an error due to the characteristic of an AD conversion circuit with high accuracy is to divide the measurement range into plural divisional ranges and store plural correction values for the respective divisional ranges. However, this method is not realistic because it is not easy to prepare a number of correction values and storing plural correction values and selecting from them results in increase in processing load.
Even if an error due to the characteristic of an AD conversion circuit can be corrected successfully, an error due to a variation of a power source voltage supplied to the AD conversion circuit cannot be corrected completely by, for example, the offsetting method using a correction value prepared in advance because it is difficult to predict a variation of the power source voltage. One method for correcting an error due to a variation of the power source voltage is to supply a highly accurate power source voltage to an AD conversion circuit. However, this method is not practical because implementing a high-accuracy power source together with an AD conversion circuit increases the cost and complicates the device configuration. In addition, since a high-accuracy power source is restricted in output current, it cannot supply power to peripheral circuits of the AD conversion circuit and hence it is inevitable to use a power source for the peripheral circuits in addition to the high-accuracy power source. It is therefore necessary to, for example, control the power supply start/stop timing of the power source for the peripheral circuits and the high-accuracy power source. As such, this method is low in practicality. As described above, the conventional methods have limits in the accuracy of AD conversion error correction and a practical method capable of increasing the correction accuracy further is desired.